Is it "Despite the persistent symptoms" or "Despite persistent symptoms"?

We use the definite article in front of a noun when we believe the listener or reader knows exactly what we are referring to:

• because there is only one: "The moon is very bright tonight." This is why we use the definite article with a superlative adjective: "He is the tallest boy in the class."

• because there is only one in that context: > "Look at the boy over there." (= the boy I am pointing at)

• because we have already mentioned it

British Council

This means that you would say "... despite the persistent symptoms ..." if there is only one set of symptoms, or a unique set of symptoms that has been defined by previous context. Because there is an unlimited set of symptoms of environmental degradation and no set of them is unique or has been defined here by context, it seems inappropriate to use "the".

If you say "... despite persistent symptoms ...", you are referring to some or any symptoms among the many that are possible. This is the appropriate usage for your sentence.

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